The present invention relates to magnetic refrigeration, magnetostrictive, and magnetoresistive materials and, more particularly, to methods of making active magnetic refrigerant regenerator alloys, and magnetostrictive and magnetoresistive materials.
Magnetic refrigeration is being considered as an alternative technique to gas compressor technology for cooling and heating based on engineering and economic considerations that indicate that magnetic regenerator refrigerators, in principle, are more efficient than gas cycle refrigerators and thus can yield savings in the cost of operation and conservation of energy.
Magnetic refrigeration utilizes the ability of a magnetic field to affect the magnetic part of a solid material""s entropy to reduce it and, therefore, increase the lattice (vibrational) part of the entropy and the temperature of the solid material in an adiabatic process. When the magnetic field is removed, the change or return of the magnetic entropy of the magnetic solid material reduces the temperature of the material. Thus, magnetic refrigeration is effected by cyclic heat dissipation and heat absorption in the course of adiabatic magnetization and adiabatic demagnetization of the magnetic solid material via application/discontinuance of an external magnetic field. A refrigeration apparatus that exhausts or vents the released heat on one side of the apparatus when the magnetic solid material is magnetized and cools a useful load on another side when the magnetic solid material is demagnetized is known in the magnetic refrigeration art as an active magnetic regenerator magnetic refrigerator (also known by the acronym AMR/MR).
U.S. Pat. No. 5,743,095 describes active magnetic refrigerant materials having general molecular formula Gd5(SixGe1xe2x88x92x)4 that provide a giant magnetocaloric effect for use in magnetic refrigerators when 0xe2x89xa6xxe2x89xa60.5. Also as described in that patent are alloys for 0.5 less than xxe2x89xa61.0 which order magnetically between 295 and 335 K and have useful and large (but not giant) magnetocaloric properties. The giant magnetocaloric effect in the former alloys (xxe2x89xa60.5) is due to a first order magnetic/structural transition, 1 and thus these alloys are useful for cooling applications from just below room temperature (295 K) down to liquid hydrogen temperatures (20 K). The large magnetocaloric effect in the latter alloys (x greater than 0.5) is due to a second order magnetic transformation, making these alloys useful magnetic refrigerants for the high temperature layer of a multilayered active magnetic regenerator of a cooling device for the rejection of heat to the ambient, and also for heat pumps to reach xcx9c350 K. The upper temperature limit can be increased from 295 K to 300 K by providing x=0.525 and heat treatment pursuant to this invention as described below such that x values of 0.525xe2x89xa6xxe2x89xa61.0 provide a large magnetocaloric effect material.
For most magnetic refrigeration and heat pump applications, large amounts (e.g. several hundred grams to hundreds of kilograms) of the magnetocaloric materials per device are needed to obtain sufficient cooling. For example, for a highly efficient magnetic air conditioner about 0.5 kg of magnetic refrigerant could provide a cooling power of 1 kW (kilowatt). Since a typical home requires about 5 kW of cooling power, about 2.5 kg of magnetic refrigerant are needed. For less efficient devices, more magnetic refrigerant material is required. The current process for making the giant and the large magnetocaloric materials Gd5(SixGe1xe2x88x92x)4 involves arc-melting the appropriate amounts of the individual elements (Gd, Si, and Ge), but this technique normally is limited to 50 to 100 gram quantities. Larger quantities can be prepared by arc-melting but generally the resulting ingots are inhomogeneous; i.e. parts of the ingot have excellent magnetocaloric properties much greater than other parts of the ingot having lower magnetocaloric properties, which at best are about the same as the current prototype magnetic refrigerant, Gd, for near room temperature applications. The ingot inhomogeneity is readily understood since, for the Gd5(SixGe1xe2x88x92x)4 alloys, the exact Gd to (Si+Ge) ratio 5:4 has been found to be critical. For example, small deviations from the 5:4 ratio have been found to lead to the appearance of the Gd(SixGe1xe2x88x92x), 1:1, or Gd5(SixGe1xe2x88x92x)3, 5:3, phases, and therefore, to significantly reduced magnetic refrigerant capacity.
These very same alloys, which exhibit the giant magnetocaloric effect, also exhibit an extremely large magnetostriction and also a large magnetoresistance when undergoing the first order transformation. Based on crystallographic data, Gd5(Si2Ge2) has a reversible linear colossal magnetostriction of xcx9c10,000 parts per million (ppm) along the [100] axis, or a volumetric colossal magnetostriction of xcx9c4500 ppm. In comparison, the magnetostriction of Terfenol-D [(Tb0.7Dy0.3)Fe2] is xcx9c1200 ppm. Since the colossal magnetostriction is due to the first order phase transition it is expected to occur in all Gd5(SixGe1xe2x88x92x)4 alloys for 0xe2x89xa6xxe2x89xa60.525. Thus the method of this invention described herein for the production of the giant magnetocaloric materials also applies for producing the colossal magnetostrictive alloys, which are useful as actuators, positioning devices, etc. controlled by the change of the magnetic field and also for magnetoelastic sensors to detect stresses.
Measurements of the electrical resistance of Gd5(SixGe1xe2x88x92x)4 alloys for 0.24xe2x89xa6xxe2x89xa60.525 as a function of temperature and magnetic field show that there is a large (xcx9c25%) change in the magnetoresistance at the first order phase transformation when induced by a magnetic field above their respective ordering temperatures. The sign of the change is positive for x=0.375 and negative for x=0.5. Such large changes have been observed in artificial, non-rare earth, magnetic multilayered materials, and have been labeled as xe2x80x9cgiantxe2x80x9d magnetoresistors. There are a number of applications in the electronics field for giant magnetoresistance materials, including read heads in magnetic recording devices and sensors.
Furthermore, the Si to Ge ratio is also important since the magnetic ordering (Curie) temperature (TC) is strongly dependent on the relative amounts of these two elements. For example, for x greater than 0.525 for the Gd5(SixGe1xe2x88x92x)4 material, the giant magnetocaloric, colossal magnetostriction, and giant magnetoresistance effects are not observed; and for x greater than or equal to 0 and less than or equal to 0.525, the Curie temperature varies almost linearly with x from approximately 20 K at x=0 to approximately 300 K at x=0.525, and the material exhibits the giant magnetocaloric, colossal magentostrictive, and giant magnetoresistance effects.
Arc-melting 100 gram quantities at a time is labor intensive and thus an extremely expensive operation. Furthermore, use of commercially pure Gd (having more than 0.1 wt. % interstitial and other impurities) instead of high purity Gd (less than 0.1 wt. % impurities) to prepare the Gd5(SixGe1xe2x88x92x)4 material for 0xe2x89xa6xxe2x89xa60.525 by arc-melting has led to only average magnetocaloric properties and not the giant magnetocaloric properties desired. The magnetocaloric properties of Gd5(SixGe1xe2x88x92x)4 for 0.525xe2x89xa6xxe2x89xa61.0, however, are not nearly as sensitive to the impurities in the Gd metal used to prepare the alloys by arc-melting. Thus there is a need for a method of producing relatively larger quantities of the Gd5(SixGe1xe2x88x92x)4 material for 0xe2x89xa6xxe2x89xa61.0 using commercially pure Gd, Si and Ge as starting charge components in a more cost effective manner.
An object of the present invention is to satisfy this need.
The present invention provides a method of making an active magnetic refrigerant, colossal magnetostrictive and giant magnetoresistive material using commercially pure rare earth, Si, and Ge as charge components by vacuum melting in a manner that enables production of relatively large amounts of the material, despite the commercially available rare earth charge component having relatively high levels of certain interstitial impurities, especially carbon impurity.
In one embodiment of the invention, the method is used to produce Gd5(SixGe1xe2x88x92x)4 material where 0xe2x89xa6xxe2x89xa60.525 exhibiting the giant magnetocaloric effect. In another embodiment of the invention, the method is used to produce Gd5(SixGe1xe2x88x92x)4 material where 0.525 less than xxe2x89xa61.0 exhibiting a large, but not giant, magnetocaloric effect.
In another embodiment of the invention, the method is used to produce magnetic materials having colossal magnetostrictive and giant magnetoresistive properties. For example, the Gd5(SixGe1xe2x88x92x)4 material where 0xe2x89xa6xxe2x89xa60.525 can be made pursuant to a method of invention.
Also, other magnetic materials including but not limited to, R5(SixGe1xe2x88x92x)4 and (R1xe2x88x92yR1y)5(SixGe1xe2x88x92x)4 materials where elements or combination of rare earth elements can be produced having giant magnetocaloric, colossal magnetostrictive, and giant magnetoresistive properties.
Heat treated magnetic refrigerant materials comprising Gd5(SixGe1xe2x88x92x)4 material where 0xe2x89xa6xxe2x89xa60.525 are provided having increased magnetocaloric properties by virtue of the heat treatment. The heat treatment can be conducted in a crucible in which the material has solidified. For example, heat treated magnetic refrigerant material comprising Gd5(SixGe1xe2x88x92x)4 material where 0xe2x89xa6xxe2x89xa60.525 is provided having at least a magnetic entropy change of 16 J/kg K.
The invention also envisions a multi-stage magnetic refrigerator or multi-layer regenerator bed having a relatively high temperature stage or layer comprising Gd5(SixGe1xe2x88x92x)4 material where x is selected between 0 and 1 and having a second relatively lower temperature stage comprising a material with a different value of x selected between 0 and 1. The value of x for the high temperature stage (or layer) is larger than the x value for the low temperature stage (or layer).
An illustrative embodiment of the invention for making Gd5(SixGe1xe2x88x92x)4 where 0xe2x89xa6xxe2x89xa60.525 involves placing stoichiometric amounts of commercially pure Gd, Si, and Ge as individual distinct charge components in a crucible, such as for example a refractory metal crucible, taking into consideration loss of charge components during melting. For example, excess Si (or Ge) may be provided to accommodate losses of Si (or Ge) during melting and heat treatment. The Gd charge component may include a relatively high C impurity content from about 0.03 to 1 atomic % as well as other impurities. The crucible charge components are heated by energization of an induction coil about the crucible to a melting temperature (e.g. about 1800 degrees C.) under vacuum (subambient pressure) at a rate that permits the alloy components to out-gas and controlling power to the induction coil when temperature of the crucible contents rises during heating due to one or more exothermic reactions between/among the alloy components. The melting temperature is held for a time sufficient to homogenize the alloy chemistry, oxidize the carbon present with oxygen in the starting Gd charge component, and prevent excessive reaction of the Si component with the refractory metal crucible. Then, the power to the induction coil is terminated to permit rapid solidification of the alloy in the crucible to avoid phase segregation. Preferably, cooling is at a rate of between 360 to 60 degrees C./minute depending upon the mass of the material to avoid phase segregation.
When the temperature of the alloy falls below the melting temperature, power is supplied to the induction coil to heat the solidified alloy at a temperature for a time (e.g. 1400 degrees C. plus or minus 10 degrees C. for 1 to 10 hours) effective to homogenize the microstructure thereof and impart enhanced magnetocaloric properties to the solidified alloy. The power to the induction coil then is terminated to allow the heat treated alloy to cool as rapidly as possible to ambient temperature to avoid eutectoid decomposition. In lieu of the in-crucible heat treatment, the solidified alloy can be removed from the crucible and heat treated in a heat treatment furnace to this same end by holding the alloys at any number of selected temperatures between 900 degrees C. and 1700 degrees C. for an appropriate length of time depending upon the temperature. For example, at 900 degrees C. the alloy should be held for 1 to 3 days, but as the heat treating temperature is increased the holding time is lowered down to one hour at 1700 degrees C. After the alloy has been heat treated at the desired temperature for the minimum holding period the alloy is cooled as rapidly as possible to ambient temperature.
The above heat treatment produces a heat treated magnetic refrigerant material that exhibits extraordinary magnetothermal properties, such as a heretofore undiscovered giant magnetocaloric effect (magnetic entropy change) based on a reversible structural magnetic and/or a reversible ferromagnetic/antiferromagnetic first order phase transition upon heating, providing a sharp reduction in magnetization near the magnetic ordering temperature (Curie temperature). For example, the heat treated alloy refrigerant exhibits a magnetic entropy change that is about 50% greater than that exhibited by the same alloy material arc-melted pursuant to U.S. Pat. No. 5,743,095.